Wireless applications rely on Radio Frequency (RF) signals to communicate information in a wireless network. A transmitter forms RF signals for transmission by modulating a baseband frequency signal (information) onto an RF carrier frequency signal. A receiver downconverts the received RF signal to the baseband frequency before baseband signal processing occurs. RF signals may be directly down converted to baseband by a zero intermediate frequency (ZIF) down converter or may be processed by one or more intermediate frequency (IF) stages before finally being converted to baseband.
The down conversion mixer may be passive or active. A passive mixer typically provides a conversion gain less than 0 dB, while an active mixer typically provides a conversion gain greater than 0 dB when down converting the received RF signal to a baseband frequency signal. Active mixers also consume more power, cause higher distortion, and have a higher noise figure than their passive mixer counterparts. The higher noise figure arises from flicker noise, which is not present in passive mixers. Flicker noise is particularly harmful for narrow band RF applications, e.g., GSM (Global System for Mobile communications).
While passive mixers provide improved linearity, noise performance, and power consumption, passive mixers tend to provide a fixed solution for all receiver configurations. For example, a passive mixer may be designed to provide a fixed conversion gain for a worst case receiver configuration. Such a fixed conversion gain results in a fixed linearity performance and unnecessarily higher power consumption for all receiver configurations. Thus, there remains a need for improved down conversion mixers.